1. Technical Field
The disclosure relates to light emitting diode lamps, and particularly to a light emitting diode lamp incorporating a protecting circuit.
2. Description of Related Art
Recently, light emitting diodes (LEDs) have come to be widely used in a variety of LED lamps, for example, miner's lamps, street lamps, submarine lamps and stage lamps, for their low power requirement and long life. The LED lamps include different numbers of the LEDs in series connection or series-parallel connection according to different illumination requirements.
It is well known that a constant current source can supply a pre-determined constant electric current to variety of loads having resistances different from each other. Since the LED is highly sensitive to electric current and substantially insensitive to voltage, the LEDs of the LED lamps are generally driven by the constant current source to reduce variation in brightness. Referring to FIG. 2, a constant current source 10 includes positive and negative input terminals IN1, IN2 for receiving electric power from an outer power source 40, positive and negative output terminals OUT1, OUT2 for supplying an electric current to an LED lamp 12 connected therebetween, a first switch K1 for connecting or disconnecting the outer power source 40 and the constant current source 10, and a second switch K2 for connecting or disconnecting the constant current source 10 and the LED lamp 12. The LED lamp 12 includes a plurality of LEDs 13 connected in series. The constant current source 10 supplies a pre-determined constant electric current to the LED lamp 12 when the first and second switches K1, K2 are turned on.
When designing the constant current source 10, a reference load between the positive and negative output terminals OUT1, OUT2 is considered to form a best application mode for the constant current source 10. For example, the reference load has a resistance equaling to that of eighteen LEDs connected in series. Therefore, the constant current source 10 is best used for supplying the pre-determined constant electric current to the LED lamp 12 which includes eighteen LEDs 13 connected in series. Referring to FIG. 3, when the constant current source 10 is used for supplying electric current to other load, such as an LED lamp 30 which includes one LED 31 and has a resistance much smaller than the reference load, the constant current source 10 may generate an impulse current at a moment when the first and second switches K1, K2 are turned on, and the constant current source 10 supplies the pre-determined constant electric current to the LED lamp 30. However, the impulse current flowing through the LED lamp 30 may cause damage to the LED 31.
It is thus desirable to provide an LED lamp with a protecting circuit which can overcome the described limitations.